Production of silicon alloys containing one or more relatively volatile metals



United States Patent 3,138,450 PRODUCTION OF SILICON ALLOYS CONTAIN- INGONE OR MORE RELATIVELY VOLATIIJE METALS Kurt Figge, Dusseldorf, FranzKaess, Traunstein, Erich Pfluger, Trostberg, Upper Bavaria, and JohannDrost, Hart (Alz), Germany, assignors to MetallgesellschaftAktiengesellschaft, Frankfurt am Main, Germany, and SuddeutscheKalkstickstotf-Werke A.'G., Trostberg, Upper Bavaria, Germany NoDrawing. Filed Mar. 24, 196i), Ser. No. 17,267 Claims priority,application Germany Mar. 26, 1959 4 Claims. (CI. 75-10) The presentinvention relates to an improved process for the production of alloys,used in iron and non-ferrous metallurgy, containing at least 15% ofsilicon which also contain one or more relatively more volatile metals,such as magnesium, calcium and the like, and may also contain othermetals, such as iron, nickel, copper, manganese, aluminum, rare earthsand the like. The production of such alloys presents certaindifliculties in view of the low boiling point of constituents thereof,such as calcium or magnesium, and their high aflinity for oxygen as theyhave a tendency to undergo uncontrollable burning off which renders itdifiicult to produce alloys of definite reproducible compositions. I

According to the invention it was found that such difliculties can beavoided in the production of such high silicon containing alloys whichalso contain relatively more volatile metals and that such alloys can beproduced with good yields upon the metal components introduced and withdefinite reproducible compositions, by producing such alloys in aninduction furnace using a crucible which primarily is constructed ofcarbon or graphite. The crucible preferably is composed of a carbontamping mass, graphite or shaped bodies of hard burned carbon or electrographite. However, crucibles constructed of graphiteclay can also beused. The use of crucibles constructed primarily of carbon has theadvantage that such materials are not attacked by the melt as was thecase with the materials previously employed for the usualsmeltin'g'furnaces, as the oxides of the alloying metals in generalbelong to the group of materials from which the lining walls of theusual smelting furnaces are constructed. As a consequence the formationof slags often occurred in View of reactions between the melt and thefurnace lining which not only made it difficult to produce alloys havingthe desired composition but also caused premature destruction of thefurnace lining. When a crucible according to the invention which isprimarily composed of carbon is used, the upper portion of the cruciblewhich does not come into contact with the melt after a period of timebecomes coated with a layer of oxides which prevent burning off of theupper portion of the crucible upon contact with oxygen of the air. Onthe other hand, such oxide coating does not disturb the composition ofthe alloy produced.

In the production of alloys containing over 15% of silicon and iron and,in addition, metals having a low melting and boiling point, such ascalcium or magnesium, it is preferable to fuse the magnesium togetherwith the remaining alloying components by supplying the magnesium to thecrucible in solid form and cover it with the remaining alloyingcomponents to prevent burning off of the magnesium during the fusion andprevent inaccuracies in the composition of the desired alloy.

It also can be expedient first to melt the magnesium in the crucible ofthe induction furnace and then to add calcium silicon, for example, atthe rate at which it is dissolved in the molten magnesium, and then toadd ferrosilicon or silicon metal in portions in such a manner that3,138,450 7 Patented June 23, 1964 the surface of the melt remainscovered with the solid material as long as possible. It was found, forexample, that molten magnesium can completely dissolve silicon alloys,such as 7090% ferrosilicon or calcium silicon, as well as granularsilicon, in a relatively short period of time at 750 C. It can beexpedient to preheat the solid alloy components before they are added,for example, with waste heat to reduce the current consumed by theinduction furnace.

One of the advantages of the process according to the invention is thattemperatures can be employed for the production of alloys ofpredetermined composition which are ZOO-300 C. below those required inthe customarily employed processes. The temperatures employed accordingto the invention do not exceed 125 0 C. The dissolution process proceedsvery quietly and the movement of the molten bath engendered by theinduction furnace is of great significance for the liquefaction as theportions which have not yet been dissolved are rinsed by the melt and acomplete homogenization is attained thereby. When the melt has beenhomogenized by the induction it should be cast in such a way that itcools and solidifies with sufficient rapidity that no separation orsegregation of the components can occur. This expediently is effected bycasting the melt in fiat chill molds.

Considerably lower losses by burning off of the lower boiling componentsof the alloys produced occur in the process according to the inventionthan when other procedur'es are employed. Furthermore, the use ofcrucible material containing substantial proportions of carbon insures asufficient heat transfer whereby local overheating is avoided andconsiderably less wear occurs. The carbon containing crucibles are moreor less conductive and therefore take up up to of the induced capacityin the case of highly conductive electro graphite and 10 to 40% in thecase of amorphous graphite, electrode tamping masses or a carbon basecontaining oxidic or siliceous additions and consequently the furnacecharge is not only heated directly by induction but also by radiationfrom the walls of the crucible. However, there is no tendency for thecrucible walls to overheat and therefore prevention of lossesthroughburning off is facilitated. In view of the homogenization of the meltsby the induction current it is not necessary to provide exteriorstirring of the melt. This materially reduces smoking and the danger ofaccidents. Previously alloys such as are produced according to thepresent invention were produced in at least two furnaces and in severalworking steps; The possibility of producing such alloys in one furnaceand in one working stepby the process according to the invention makesconsiderable savings in current possible and therefore a simplificationand a reduction in the costs of the process.

Instead of proceeding in the previously described manners, it is alsopossible to produce magnesium containing melts by pouring the alloyingcomponents, such as ferrosilicon, calicum silicon and/0r silicon metalor mixtures thereof in molten form over the magnesium in the inductionfurnace in solid or liquid form and subsequently homogenizing themixture.

In some instances it may be expedient to melt some or all of thealloying components in the carboniferous crucible of the inductionfurnace by resistance heating and then to homogenize the melt byinduction heating.

It also was found advantageous in some instances when the induction coilfor the induction furnace is not one continuous winding but rather issubdivided and each portion connected individually to the transformer.In this way it is possible to supply strong inductive forces to thoseportions of the crucible which must take up the greatest quantities ofenergy.

The process according to the invention fundamentally can be carried outin induction furnaces at frequencies between about 50 and 10,000 Hertz(c.p.s.). The intensity of the movement of the molten bath in thecrucible can be considerably influenced by the selection of thefrequency of the heating current. The preferred intensity can bedifferent for different types of alloys. The stronger the bath movementthe greater the danger of oxidation by the oxygen of the atmosphere, theweaker the bath movement the slower the progress of the dissolution andhomogenization. Preferably a powerline frequency induction furnaceoperating at, for example, a frequency of 50 Hertz of a rated capacity450 kw. and a rated voltage of 400 volts is employed for the processaccording to the invention.

The process according to the invention is especially adapted for theproduction of the alloys of US. Patent 2,837,422, namely, pre-alloysessentially composed of 15- 50% of magnesium, 2-10% of calcium, 35-60%of silicon and the remainder iron, which in addition can contain up to2% of rare earths.

The following examples will serve to illustrate several embodiments ofthe invention.

Example 1 91 kg. of magnesium bars were introduced into an emptypreheated crucible produced from a carbon tamping mass and covered with40 kg. of CaSi and 170 kg. of coarse pieced 75% FeSi and 3.2 kg. ofcerium misch metal. The induction furnace employed for the heating wasone operating at a frequency of 50 Hertz and having a rated capacity of450 kw. and a rated voltage of 400 volts. The electric current wasturned on and the contents of the crucible melted down in 40-45 minutes,care being taken that the temperature did not exceed 1250 C. After thecontents had been liquefied they were homogenized for minutes at a lowercurrent load. After the current was turned off the crucible was tippedand emptied in about 1 minute into a pan having a surface area of 2000 x1400 mm. which was clad with cast iron plates 50 mm. thick. The heightof the alloy in the pan was only about 30-40 mm. and it solidified in amaximum of 1-2 minutes in view of the rapid heat transfer to the base sothat a segregation of the alloy components was avoided. The alloy plateformed, after completely cooling down, was broken up to the desiredsized pieces.

Example 2 91 kg. of mganesium bars were placed in an empty crucible asin Example 1 and melted down in -15 minutes at a frequency of 50 Hertz.The temperature of the resulting molten magnesium was about 750 C. Then100 kg. of pieces of 75% FeSi were added. As soon as the cruciblecontents started to liquefy a further 70 kg. of 75% FeSi were poured in.Then a layer of 40 kg. of CaSi (about 30% Ca) was added over the top.After complete dissolution, which was attained in about 35-40 minuteswhile taking care that a temperature maximum of 1250" C. was notexceeded by regulating the current load on the crucible, the cruciblecontents were homogenized for 5-10 minutes. After the current was turnedoff the contents were completely emptied in about 1 minute by tippingthe crucible and rapidly solidified.

Example 3 93 kg. of magnesium bars were introduced into an emptycrucible as used in Example 1 and a liquid mixture of 40 kg. of CaSi and170 kg. of 75 FeSi poured thereover. The current was turned on and themixture homogenized by induction for about 10 minutes, care being takenthat the temperature did not exceed 1250 C. After the current was turnedoff the crucible was tipped and emptied in about 1 minute. Analogousresults were obtained when the magnesium bars were first melted in thecrucible before adding the remaining alloying components.

4 Example 4 An induction furnace with an operating frequency of 50 Hertzand a rated capacity of 450 kw. and a rated voltage of 400 volts similarto that used in the previous examples was employed except that itsinduction coil was provided with two separate current connections.

First a manganese silicide alloy containing 50% manganese and 50% ofsilicon was prepared by fusing a mixture of such metals in the preheatedcarbon crucible of such furnace. The melting point of such mixture wasabout 1130 C. The resulting alloy was poured out of the crucible,solidified rapidly in a chilled mold and broken into small pieces (max.size 10 mm.).

Then 34 kg. of Mg- Si were produced in the same crucible by meltingmagnesium bars and adding the required quantity of silicon. Gnly thelower portion of the coil was activated while such alloy was prepared toprevent overheating of the portion of the crucible not covered by themelt. When the crucible charge had melted completely 66 kg. of thepreviously prepared fine piece manganese silicide were added in portionsand the temperature of the charge slowly raised until such additionswere completely dissolved. After the last addition had melted the upperportion of the coil was activated and the charge homogenized for 5minutes, care being taken that its temperature did not exceed 1250 C.The crucible was tipped and emptied in about 1-2 minutes into a chilledmold where the alloy was rapidly solidified. The composition of theresulting alloy was 20% Mg, 33% Mn and the remainder Si.

Example5 10 kg. of Mg bars and 56 kg. Al bars were placed in a preheatedcarbon tamping mass crucible in a furnace as in Example 1 and melteddown. Then 39 kg. of CaSi (31% Ca, 61% Si and 8% Fe) were added and thecrucible contents liquefied by slowly raising the temperature. After thecharge had liquefied it was homogenized for a further 5-10 minutes andwas then tipped out in 1 minute and rapidly solidified in a chilledmold.

The composition of the resulting alloy was about 10% Ca, 10% Mg, 20% Si,56% Al and the remainder Fe.

Example 6 91 kg. of Mg bars were placed in a crucible as used in Example1 and melted down in about 10 minutes at a frequency of 50 Hertz. Thetemperature of the liquid Mg was about 750 C. Then 40 kg. of CaSi (about31% Ca, 61% Si and 8% Fe) were added and dissolved by slowly increasingthe temperature. Then the melt was covered with 100 kg. of pieces of 75%FeSi and, in the measure in which such FeSi was dissolved, a further 70kg. of FeSi was spread on so that the liquid crucible contents werecovered as long as possible by the FeSi which had not yet dissolved. Thelast residues of the FeSi dissolved in about 30-35 minutes, care beingtaken that the temperature did not exceed 1250 C. during suchdissolution. Thereafter the crucible contents were homogenized for afurther 5 minutes and after the current was cut off they were poured outin 1 minute into a chilled mold.

Example 7 50 kg. of Al pigs were introduced into a preheated carbontamping mass crucible and covered with 150 kg. of small pieced CaSi(max. 50 mm., Ca=32%, Si=60% and Fe=8%) and this in turn covered withkg, of silicomanganese (70.6% Mn, 19% Si and remainder Fe). The cruciblewas contained in an induction furnace as in Example 1 which, however,was also provided with graphite rod resistance heating. The contents ofthe crucible were first melted down by turning on the resistanceheating. When the major portion of the crucible contents were liquid,the resistance heating was turned off and the induction heating turnedon to effect the complete liquefaction and homogenization of thecrucible contents. The

contents were then rapidly poured into a flat open pan lined with carbonblocks. The composition of the resulting alloy was about 16% Ca, 17% Al,36% Si, 23% Mn and the remainder Fe.

We claim:

1. A process for the production of homogeneous silicon containing alloyscontaining at least 15% of silicon and at least one metal selected fromthe group consisting of magnesium and calcium which comprises meltingthe components of the alloy in a crucible produced from carboncontaining material in an induction furnace, homogenizing the resultingmelt by induction and casting and cooling the alloy to solidify itbefore segregation of the components can take place therein.

2. A process for the production of an alloy containing magnesium,calcium, iron and at least 15% of silicon which comprises introducingmagnesium in solid form into a crucible produced primarily from carbon,covering such solid magnesium with the remaining alloying components,induction heating such alloying components to melt them down at atemperature up to 1250 C., homogenizing the resulting melt by inductionand casting and cooling the alloy to solidify it before segregation ofthe components can take place.

3. A process for the production of an alloy containing magnesium,calcium, iron and at least 15 of silicon which comprises introducingmagnesium in solid form into a crucible produced primarily from carbon,induction heating said magnesium to melt it down, gradually addingcalcium silicon to said molten magnesium at the rate at which it isdissolved, then covering the surface of the melt with solidferrosilicon, heating the crucible charge by induction to effect fusionof the ferrosilicon and adding further quantities of solid ferrosiliconto maintain coverage of the surface of the melt with solid ferrosiliconas long as possible, after the last ferrosilicon has fused homogenizingthe melt by induction, the temperature of the crucible contents duringfusion and homogenization being up to 1250 C., and casting and coolingthe alloy to solidify it before segregation of the components can takeplace therein.

4. A process for the production of an alloy containing magnesium,calcium, iron and at least 15% of silicon which comprises introducingmagnesium in solid form into a crucible produced primarily from carbon,induction heating said magnesium to melt it down, gradually addingcalcium silicon to said molten magnesium at the rate at which it isdissolved, then covering the surface of the melt with solid silicon,heating the crucible charge by induction to effect fusion of the siliconand adding further quantities of solid silicon to maintain coverage ofthe sur face of the melt with solid silicon as long as possible, afterthe last silicon has fused homogenizing the melt by induction, thetemperature of the crucible contents during fusion and homogenizationbeing up to 1250 C., and casting and cooling the alloy to solidify itbefore segregation of the components can take place therein.

Introduction to Magnesium and Its Alloys, page 51, Edited by Alico.Published in 1945 by the Ziff-Davis Publishing Co., New York.

1. A PROCESS FOR THE PRODUCTION OF HOMOGENEOUS SILICON CONTAINING ALLOYSCONTAINING AT LEAST 15% OF SILICON AND AT LEAST ONE METAL SELECTED FROMTHE GROP CONSISTING OF MAGNESIUM AND CALCIUM WHICH COMPRISES MELTING THECOMPONENTS OF THE ALLOY IN A CRUCIBLE PRODUCED FROM CARBON CONTAININGMATERIAL IN AN INDUCTION FURNACE, HOMOGENIZING THE RESULTING MELT BYINDUCTION AND CASTING AND COOLING THE ALLOY TO SOLIDIFY IT BEFORESEGRETATION OF THE COMPONENTS CAN TAKE PLACE THEREIN.